1. Technical Field
The present disclosure relates to a management apparatus for controlling a rotating motor, in particular a spindle motor, and of an annexed load, in particular a voice coil motor, during power loss.
2. Description of the Related Art
Hard disks for computers or other electronic devices which comprise one or more magnetic disks for storing data and a magnetic head for reading/writing disks are well known in the state of the art. The disk is assembled on a spindle motor and both the spindle motor and the reading/writing head are controlled by a control device, preferably integrated.
When the hard disk is powered off under normal shutdown conditions, the reading/writing head is arranged in a safety area; the operation is generally called parking the head. The parking is performed by a so-called voice coil motor (VCM).
If it occurs that, during a sudden power loss, the reading/writing head is not parked in the specific area, it may fall on the disk and damage it.
When a sudden power loss occurs, the control device may provide energy for a limited period of time by converting, for example, the kinetic energy of the spindle motor of the hard disk which is still rotating into electric energy.
The energy provided by the kinetic power of the spindle motor, decreased by the energy dissipated by the mechanical frictions of the motor, is available to the VCM as electric power through the control device. The energy available for the VCM is the energy provided by the spindle motor less the energy dissipated by the resistances of the MOS transistors of the control device and the energy dissipated in the motor coils.
The energy provided by the spindle motor is due to the back-electromotive force (BEMF) which, once rectified, provides energy to the VCM motor which acts as an electric load. The BEMF has a sinusoidal form whose frequency is p times the rotation frequency of the motor where p is the number of pairs of rotor poles. The BEMF developed by the spindle motor phases are to be converted into a direct voltage before being provided to the VCM. This may occur by means of the body diodes of the MOS transistors of the control device.
To decrease the dissipation of the body diodes, synchronous rectification technology is currently used. This technology provides switching, under the conditions of turn-on and turn-off, the MOS transistors of the control device in an appropriate sequence; thereby the rectification process is provided with less energy dissipation through the MOS transistors given that the dissipative portion is only depicted by the on resistance Rdson of the transistors. The voltage Vm for driving the VCM is greater than the preceding case.
FIG. 1 shows a circuit scheme of a system 100 for hard disk in which a spindle motor 105 acts as generator during a sudden power loss. The VCM or load 150, absorbs the current provided by the BEMF, BEMFA, BEMFB, BEMFC generated by the spindle motor 105 so that a voltage Vm is provided at the ends of the load 150. The driver of the spindle motor 105 comprises transistor half-bridges M1-M2, M3-M4, M5-M6 controlled by a controller 108 and belonging to an integrated circuit 107; each transistor belonging to the half-bridges is equipped with its body diode.
At every instant, the high side transistor that is turned on by the controller 108, belongs to the half-bridge connected to the coil of the motor whose BEMF has a greater amplitude than the other two BEMFs in the case of three-phase spindle motor, while at every instant, the low side transistor that is turned on belongs to the half-bridge connected to the coil of the spindle motor whose BEMF has a lesser amplitude than the other two BEMFs. The on/off configuration of the transistors changes every 60 electrical degrees.
The voltage Vm is a direct voltage with a residual ripple, as shown in FIG. 2.
The ripple frequency is six times the BEMF frequency and its peak amplitude is approximately 1.7 for the BEMF peak.
When the VCM is connected to the voltage Vm, it may be considered to a first approximation as a simple resistor Rvcm. The resulting rectifying circuit, in which the resistive load Rvcm is connected to the voltage Vm within 60 degrees, is shown in FIG. 3. The resistances Rdson are the on resistances of the transistors M1 and M4 while the resistances Rm and the inductances Lm are those of the two coils of the spindle motor 105 associated with the BEMF, BEMFA, BEMFB; a current Ivcm flows in the circuit.
The power available to the VCM is a small portion of the kinetic energy available by the spindle motor. If said energy is not enough to complete the parking operation of the head before the voltage Vm reaches the minimum threshold for the operation, the remaining energy is to be supplied to the VCM. This involves a certain energy equalization which may be obtained by means of an impedance control between the spindle motor and the VCM motor.
In most cases the resistance of the spindle motor is lower than the resistance of the VCM and said impedance decoupling prevents the maximum transfer of energy from occurring.
Patent application US 2010/0165811 describes a way to obtain an impedance matching between the spindle motor and the VCM motor. This allows extending the time period in which the voltage Vm generated by the spindle motor remains above the minimum threshold Vth, i.e., the minimum voltage to allow the integrated control device to operate.
US 2010/0165811 describes a system which, by conveniently controlling the driving signals of the MOS transistors connected to the coils of the spindle motor, may increase the period of time in which the voltage Vm remains above the voltage Vth. The circuit in FIG. 3 may be transformed into the circuit in FIG. 4, i.e., the circuit in FIG. 4 is equivalent to the one in FIG. 3, in which R10=2*Rm+2*Rdson and R20=Rvcm+2*Rdson is indicated in the portion of 60 electrical degrees considered, and in which there is a switching circuit consisting of the half-bridge having the transistor M1 as high side and the transistor M4 as low side, which are driven with one duty cycle D and one 1-D, respectively. The circuit in FIG. 4 comprises a capacitor Cm for filtering the ripple and a current generator Invc, which represents other devices connected to the VCM, e.g., a memory reading/writing channel, and at the ends of which there is the voltage Vm. By applying Thevenin's Theorem to such a circuit in FIG. 4, the circuit in FIG. 5 is obtained. A voltage generator Vbemf/D is applied to a resistance R10/D2 and to a parallel of another resistance R20 and the current generator Invc. One can obtain the maximum power transfer by making the resistance R10/D2 to be equal to the impedance due to the parallel of R20 and Invc and the following is obtained:
  D  =                    R        ⁢                                  ⁢        10        *        Invc            +                                    R            ⁢                                                  ⁢                          10              2                        *                          Invc              2                                +                                                    Vbemf                2                            *              R              ⁢                                                          ⁢              10                                      R              ⁢                                                          ⁢              20                                            Vbemf  
If Invc=0, i.e., if the energy absorbed by other devices connected to the VCM, e.g., a memory reading/writing channel, is negligible, the perfect impedance matching is obtained when
  D  =                              R          ⁢                                          ⁢          10                          R          ⁢                                          ⁢          20                      .  
Again, in accordance with said prior art, the best strategy for keeping the voltage Vm higher than the threshold voltage Vth as long as possible is the one of starting the driving of VCM by supplying to it a voltage Vm and imposing D=1, i.e., with synchronous rectification, once the hard disk is off due to sudden power loss. The duty cycle D is gradually decreased from 1 to its optimum value
  D  =                    R        ⁢                                  ⁢        10                    R        ⁢                                  ⁢        20            each time the voltage Vm reaches the minimum threshold Vth from its maximum value.